Neutronic reactor control element



Sept. 13, 1960 Filed Aug. 20, 1953 11 Sheets-Sheet 1 l INVENQTOR.

BY Henry W. Newson ATTORNEY Sept. 13, 1960 H. w. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT 11 Sheets-Sheet 2 Filed Aug. 20, 1953 INVENTOR.

BY Henry W. Ne wson ATTORNEY Sept. 13, 1960 Filed Aug. 20, 1953' H. W. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT 11 Sheets-Sheet 3 Acfive Porfion or Core 10 IN VEN TOR.

Henry W. Newson f' -q/ 44-M ATTORNEY Sept. 13, 1960 141w. NEWSON 2,9525% NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953 ll Shets-Sheet 4 INVENTOR.

2 5- BY Henry w. Newson Sept. 13, 1960 H. W. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953 ll Sheets-Sheet 5 IN V EN TOR.

BY Henry W. Newson ATTORNEY Sept. 13; 1960 H. w. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT ll Sheets-Sheet 6 INVENTOR.

BY Henry W. Newson ATTORNEY Sept. 13,

H. W. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953 11 Sheets-Sheet '7 INVENTOR.

BY Henry W; Newson ATTORNEY Sept. 13, 1960 H W. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953 11 Sheets-Sheet 8 IN V EN TOR.

BY Henry W. Newson ATTORNEY Sept. 13, 1960 H. w. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Fild Aug. 20. 1953 11 Sheets-Sheet 9 1 BY Henry W. Newson flw/Qfim ATTORNE Y Sept. 13,1960 H. w. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953 11 Sheets-Sheet 1o FllilEL INVENTOR Henry W N w A TTOP/VE Y Sept. 13, 1960 H. w. NEWSON NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1955 11 Sheets-Sheet 11 IN V EN TOR.

BY Henry W. Newson ATTORNEY United States NEUTRONIC REACTOR CONTROL ELEMENT Filed Aug. 20, 1953, Ser. No. 382,498 1 Claim. or. 204-1932 The present invention relates generally to the neutronic reactor art, and it is particularly concerned with novel control elements especially adapted for use in association with extremely high flux reactors, such as the reactors described in co-pending US. patent applications Serial No. 314,595, now Patent No. 2,831,806, and Serial No. 360,190, now Patent No. 2,832,732, of common assignee, both entitled Neutronic Reactor, filed October 14, 1952, and June 8, 1953, respectively, in the name of Eugene P. Wigner. The present application constitutes a continuation-in-part of US. patent application Serial No. 366,615, now Patent No. 2,872,399, of common assignee, for Self-Reactivating Neutron Source for a Neutronic Reactor, filed on July 7, 1953, in my name.

As used in this specification and in the appended claim, the following terminology is defined as indicated below:

Thermal neutronsNeutrons having a substantially Maxwellian number-energy distribution characteristic about an energy value equal to CT, Where C is a con stant and T is the temperature in degrees Kelvin. (CT=0.025 electron volt at 15 C.)

Slow neutrons-Neutrons having a kinetic energy less than one electron volt.

Fast neutrons-Neutrons having a kinetic energy greater than 100,000 electron volts.

Intermediate neutronsNeutrons having a kinetic energy in the range between that of fast neutrons and that of slow neutrons.

Reactor active portion (core)That inner portion of a neutronic reactor which contains fissionable material and is characterizedby a multiplication constant (k) greater than unity. The symbol (k...) is sometimes employed in the literature to represent the multiplication constant (k).

FissionThe splitting of an atomic nucleus, upon the absorption of a neutron, into a plurality of fragments of greater mass than that of an alpha particle, the splitting being accompanied by the release of energy and a plurality of neutrons.

FissionableHaving the property of undergoing fission upon the absorption of a slow neutron.

FertileHaving the property of converting to fissionable material upon the absorption of a slow neutron.

Moderator material-A non-gaseous material for which the ratio is greater than 10, wherein E is the average loss in the Diluent material-Any non-fissionable material present in the active portion of a neutronic reactor.

DilutionThe ratio of diluent atoms to fissionable atoms in the active portion of a neutronic reactor.

Specific activityThe number of disintegrations per second per unit volume of radioactive material.

Specific powerKilowatts heat output of a neutronic reactor per kilogram of fissionable material present in the active portion.

As is now well known, by massing together sufiicient fissionable material under appropriate conditions, a selfsustaining neutron reactive assemblage may be formed, which assemblage, by reason of its ability to generate neutrons at an equal or greater rate than they are being lost thereto by absorption or leakage, is capable of maintaining a self-sustained chain reaction of neutron induced fission. Apparatus which employs such a neutron reactive assemblage has been termed a neutronic reactor, 7

Experimental Production of a Divergent Chain Reaction, E. Fermi, Am. Jour. of Physics, vol. 20, No. 9, December 1952. Detailed descriptions of the theory and practice of the design, construction, and operation of neutronic reactors generally are set forth in the Science and Engineering of Nuclear Power, C. Goodman, Addison Wesley Press, Inc., Cambridge, Massachusetts, v01. 1 (1947), and vol. 2 (1949); in The Elements of Nuclear Reactor Theory, S. Glasstone and M. Edlund, D. Van Nostrand Company, Inc., New York, 1952; in Elementary Pile Theory, H. Soodak and E. C. Campbell, John Wiley and Sons, New York, 1950; and in the co-pending U.S. patent application Serial No. 568,904, now Patent No. 2,708,656, of common assignee, filed December 19, 1944, in the names of E. Fermi and L. Szilard. Reference is made particularly to Chapters 4, 5, 6, 8 and 9 of Goodman, vol. 1. Any terminology not specifically defined herein is used in the sense defined on pages 112 to 115 of Goodman, vol. 1.

One of the important uses of neutronic reactors is to irradiate materials with neutrons and other nuclear particles and radiations. In this manner, radioactive isotopes may be produced for all chemical elements with the exception of helium. The physical transformation of elements as a result of irradiation in a neutronic reactor may be accomplished through any one of a number of reactions which are fully described in the published literature. Radioactive isotopes produced by neutronic reactors are receiving large commercial interest, particularly such isotopes as H C P S and 1 As a result, there is a great demand for neutronic reactors For various reasons, as fully presented in the aforementioned co-pending applications of Eugene P. Wigner, it is particularly desirable that a reactor intended to be used for the neutron irradiation of materials be capable of operation at extremely high values of slow neutron flux. The above-mentioned co-pending applications of Wigner disclose slow, or thermal, reactors especially adapted for neutron irradiation, utilizing beryllium in the reflector region thereof, and operating at a design power level of about 30,000 kw., a peak slow neutron flux of about 5 10 neutrons/cm. /sec., and an average slow neutron flux throughout the active portion of about 2X 10 neutrons/ cm. sec.

Such reactors, by Virtue of their extremely high flux, present certain unique control problems and diificulties. For example, for reasons which will hereinafter be described in detail, the inherent reactivity of the reactor, per se, varies over a wide range depending upon particular conditions existing at any given time, e.g., the temperature, the extent of depletion of the fuel, the amount to accumulated fission products, the number and type of irradiations being performed, etc. Hence, the control elements provided for such a reactor must be capable of controlling an unusually large range of excess reactivity in order to compensate for the wide variations in the inherent reactivity of the reactor.

Another unique feature of the reactors described in the above-referred to Wigner applications is that the slow neutron flux is actually higher in the reflector region thereof than it is in the active portion, or core, thereof, and this feature too has a significant bearing on the control problem.

Accordingly, it is an object of the present invention to provide a control element having a large range of reactivity control and being especially adapted for control of a reactor having a very high slow neutron flux.

Another object of the invention is to provide a control element especially adapted for the control of a reactor having a higher slow neutron flux in its reflector than in its active portion.

In accodance with the principles of the present invention, there is provided an elongated control element having a plurality of discrete longitudinally extending sections, these sections being individually disposable in the reactor and differing markedly from one another in their neutron reactive characteristics, and at least one of the sections being formed essentially of moderator material. In the preferred form of the invention, the control element is positioned for movement through the reflector region of the reactor at a point adjacent the active portion and near the point of maximum neutron flux in the reflector, and consists of two sections, the non-moderator section containing a slow neutron absorber, such as boron or cadmium.

The many advantages of neutronic reactor control elements constructed according to the teachings of the present invention will become readily apparent from a study of the following description of the invention, together with the illustrative embodiment shown in the drawings, in which:

Figure 1 is a vertical central sectional view of a neutronic reactor having incorporated therein the novel control elements of the present invention;

Figure 2 is a horizontal sectional view taken along line 22 of Figure 1;

Figure 3 is a vertical central sectional view of the upper portion of the reactor tank shown generally in Figures '1 and 2, this view being taken looking from left to right in Figure 2;

Figure 4 is a vertical central sectional View of the remaining portion of the reactor tank shown in Figure 3;

Figure 5 is a fragmentary sectional view of a detail of Figures 3 and 4;

Figure 6 is a horizontal sectional view taken along line 66 of Figure 4; V

Figure 7 is a horizontal sectional view taken along line 77 of Figure 3;

Figure 8 is an isometric view of one of the fuel assemblies of the neutronic reactor;

Figure 9 is an expanded transverse sectional view of the fuel assembly taken along line 99 of Figure 8;

Figure 10 is a fragmentary expanded sectional view of a detail of the fuel assembly shown in Figure 9;

Figure 11 is a vertical central sectional view of the upper portion of one type of composite control element shown generally in Figures 3 and 4, this type of control element being constructed according to the principles of the present invention; 7

Figure 12 is a vertical central sectional view of the remaining portion of the composite control element shown in Figure 11;

Figure 13 is a vertical central sectional view of the upper portion of a second type of composite control element shown generally in Figures 3 and 4;

Figure 14 is a vertical central sectional view of the remaining portion of the control element shown in Figure 13;

Figure 15 is a schematic horizontal sectional diagram of the mid-plane of the core and preliminary reflector of the reactor illustrating the loading pattern employed in the specifically exemplified reactor;

Figure 16 is a vertical central sectional view of the elements embodying the principles of the present invention;

Figure 20 is an elevation view, partly in section, of

the reflector assembly schematically indicated in Figure 15;

Figure 21 is a plan view of the reflector assembly shown in Figure 20; and

Figure 22 is a transverse sectional view taken along line 22-22 of Figure 20.

As shown in Figures 1 and 2, the reactor has a rectangular central channel 20, which will be understood to contain the active portion or core 10 and the preliminary reflector 12, shown in Figure 15.. The central channel 20 is surrounded by a first reflector 22 formed of moderator material, both the reflector 22 and the central channel 20 being disposed within a tank 24. As will hereinafter be described in detail, ordinary water circulates downwardly through tank 24 and occupies all space within the tank not otherwise occupied, the water serving as both moderator and coolant for the reactor. A second reflector 26 also formed of moderator material is disposed about the outer periphery of the tank 24, and a thermal shield 28 surrounds the second reflector 26. A massive biological-radiation shield 30 surrounds the thermal shield 28. Figures 1 and 2 also show a number of radiation passages 32 extending through the biological shield 30, the thermal shield 28, the second reflector 26, and the first reflector 22. to the central channel 20 of the reactor. These passages are provided to enable operating personnel to position materials which are to be irradiated at a desired position relative to the core of the reactor, and they may be plugged with shielding material when not in use.

In the particular construction of the neutronic reactor which will be used as a specific example throughout this application, the first reflector 22 is constructed of elongated rectangular beryllium blocks, and it forms a. cylinder approximately 54 inches in outside diameter and 40 inches high. The rectangular central channel 20 formed in the first reflector is approximately 16-inches wide by 28 inches long. The active portion or core 10 is centrally located vertically with respect to the central channel 20 and has a height. of 23% inches. The active portion 10 is substantially coextensive in length with the central channel 20 but occupies only the first threefifths of the width thereof (from left in Fig. 1). The remaining two-fifths of the width of the central channel 20 is occupied by a preliminary reflector 12 of beryllium which is substantially coextensive in height with the first reflector 22. As can be seen in Figure 15, and as will hereinafter be more fully described, the preliminary reflector 12 is made up of beryllium reflector assemblies 150, shown in detail in Figures 20', 21 and 22, andthe beryllium moderating elements 96 of reflecting control elements 82, shown in detail in Figures 11 and 12... The preliminary reflector 12 and the first reflector 22 are provided with a multitude of vertically extending 51 inch diameter coolant holes 98 (Figures 6, 11, 12, 20, 21 and 22) and 116 (Figure 6), respectively, the number and spacing of these holes being such that the volume percentage of water in the preliminary reflector 12 is about 8%, and in the first reflector 22 about 4%. The tank 24 is one inch thick, is formed of aluminum, and extends some 19 feet above the active portion and some 8 feet below the active portion 10. The second reflector 26 is formed of graphite, has a height of about 9 feet, and is centered in a vertical direction with respect to the active portion 10. In cross section, it forms a rectangle about 12 feet by 14 feet. This graphite reflector is divided into a region 27 nearest to the active portion 10 consisting of about 700,000 graphite balls 33, one inch in diameter, and a region 31 of substantially solid graphite.

Region 27 has a square cross section about 7 feet, 4 inches on a side. It will be appreciated that since region 27 is in a relatively high flux portion of the reactor, considerable distortion in this region would normally be anticipated due to the Wigner effect. By utilizing graphite balls in this region any expansion due to radiation effects or temperature increase is readily accommodated by a corresponding slight variation in the level of the balls. Furthermore, should the balls become badly deteriorated from the Wigner effect, they are readily removable by gravity via chutes 151 and replaceable from the top of the reactor. The region 31 of substantially solid graphite has a multitude of one-half inch diameter coolant holes (not shown) drilled vertically therethrough, the spacing of the holes increasing outwardly such that the volume occupied thereby varies from about four percent at the inner edge of region 31 to zero (no holes) in the outer half of region 31. The thermal shield 28 comprises two spaced 4 inches thick layers of steel, the purpose of this shield being to absorb most of the residual radiation and thus protect the inner portion of the biological shield 30 from overheating. The biological shield 30 is about 9 feet thick and is formed of a barytes concrete in which the gravel part of the mix is approximately 93% BaSO The entire reactor, including shield 30, thus forms an approximate cube about 34 feet to a side.

As previously indicated, the neutronic reactor is both cooled and moderated by the same flow of water. The tank 24 is provided with an enlarged reservoir portion 112' near the top thereof. Coolant inlet pipes 114 are connected to the reservoir portion 112 of the tank 24 and introduce the water coolant into the reservoir portion 112 of the tank 24. The water coolant then flows downwardly through the coolant channels in the active portion 10, as will later be described, and also through the holes 98 and 116 in the preliminary reflector 12 and the first reflector 22. Return pipes 118 adjacent to the bottom of the tank 24 return the coolant upwardly through the shield 30. The water is then recirculated downwardly through another portion of the shield 30 through coolant exit pipes 120 disposed in the shield 30. In this manner, all portions of the reactor located .Within tank 24 are cooled. The exit water is cooled and demineralized by conventional equipment, not

} shown, located externally of the shields and is returned to the reactor via inlet pipes 114.

Tank 2'4 is provided with a removable bellows type expansion joint section 156 interconnecting its enlarged reservoir portion 112 and its lower smaller portion, this joint section having a plurality of convolutions of thin stainless steel. This expansion joint section permits free thermal expansion and contraction of the tank 24 without setting up excessive stresses in the structure. Removal of this section 156 provides access to the graphite ball containing region 27 for filling same when it is desired to replace the graphite balls.

The water in tank 24 directly above and below the active portion forms a pool 124 which also serves as within tank 24.

A secondary air coolantsystem is provided for cooling portions of the reactor located outside of the tank 24. The cool air enters near the top of the reactor at 152 and travels downwardly and inwardly through thev .shield 30 via ducting 153 to annular manifold 154. It

then flows in two parallel paths to the annular air space 155 which is left between the top thermal shield 23 and the top of the second reflector 26. The first path is by way of the horizontal space between the two layers forming the top thermal shield 28. The second path is first downwardly through the vertical space between the two layers forming the side thermal shield 28, and then upwardly through the interstices between the graphite balls of region 27 of the second reflector 2'6 and through the multitude of small holes drilled for this purpose through the solid graphite region 31 of the second reflector 26. The air is then removed from space 155 by way of ducts, not seen in Fig. 1, which pass downwardly and outwardly through the shield 30.

The active portion 10 of the reactor is formed by means of a plurality of fuel assemblies 34 (Figures 8 through 10) immersed in the water moderator. The fuel assemblies 34 are supported between an upper assembly grid 36 and a lower assembly grid 38, as shown in Figures 3 through 5. A lower support member 40 is secured to the tank 24, and secures the lower assembly grid 38. The upper assembly grid 36 is also secured to the tank 24 by an upper support member 42. An upper guide grid 44 rests upon the upper assembly grid 36 by means of a grid spacer 46. A lower guide grid 48 is positioned beneath the lower assembly grid 38 and is attached to the tank 24 by the support member 40. The upper guide grid 44, grid spacer 46, and upper assembly grid 36 together form a unit which may be locked in place in the upper support member 42 by locking mechanism 168 or may be removed as a unit from the upper support member 42.

The fuel assemblies 34 are specifically illustrated in Figures 8 through 10. Fuel plates 50, containing fissionable material as a centrally positioned inner layer thereof, are provided with a corrosion resistant cladding 54 and are secured to the side plates 52. The fuel plates 50 are curved or bowed somewhat for reasons hereinafter to be discussed. A pair of comb-shaped supports 62 at the ends of the plates 50 aid in maintaining the plates in rigid spaced relationship. The fuel assemblies 34 terminate in hollow aluminum end boxes 58 and 60 which permit the fuel assemblies to be secured between the upper and lower assembly grids 36 and 38.

In the particular construction of the neutronic reactor used as an example throughout this description, the inner layer of plates 50 is formed of uranium alloyed with aluminum, the alloy containing 14% uranium by weight. The uranium employed in the alloy is enriched in its fissionable U isotope to such a degree that 93.5% of the uranium atoms are U atoms. All other portions of the fuel assembly are formed of aluminum. Each fuel assembly 34 contains eighteen plates 50, each plate being 2.85 inches wide, before curving. The sixteen inner plates 50 are 24% inches long and the two outer plates are 28 /8 inches long. Each of the plates 50 is 0.06 inch thick, including the cladding 54, and the spacing between the surfaces of adjacent plates is 0.118

is 2.55 inches wide, before curving, 23% inches long,

7 and 0.021 inch thick. Each of the plates 50 contains 7.7 grams of U the entire fuel assemblies 34 each containing approximately 140 grams of U The side plates 52 and comb-shaped supports 62 are in contact only with the fissionable free border of plates 50 so that nothing except the coolant water contacts the plate surfaces at any point overlying the 2.55 inches by 23% inches fissionable layer. The side plates are 0.118 inch thick, and have the same length, 28% inches, as the two outer fuel plates. The end boxes- 58 and 60 are attached to the ends of the two outer fuel plates 50 and to the ends of said plates 52.

Figure 7 indicates that the upper assembly grid 36 is. provided with rows of circular apertures 64, some of the circular apertures 64 being separated by eight apertures 66 which are essentially rectangular but have a curved side which conforms to the bowed shape of one side of the central section 161 of control elements 82 and 84 hereinafter to be described. The upper end boxes 60 of the fuel assemblies 34 are provided with circular connectors 68 which are adapted to fit into the circular apertures 64 of. the upper assembly grid 36. The lower assembly grid 38, illustrated in Figure 6, is provided with rows of rectangular orifices 70, aligned with the circular apertures 64 of the upper assembly grid, some of the orifices 70 being separated by eight orifices 72, which have the same shape as the eight apertures 66 in the upper assembly grid 36 and are aligned therewith. The lower end boxes 58 of the fuel assemblies 34 have rectangular connectors 69 which are adapted to snugly fit the rectangular orifices 70 of the lower assembly grid 38, thus permitting fuel assemblies 34 to be secured between the upper and lower assembly grids 36 and 38.

As indicated in Figure 15, the preliminary reflector 12 of the reactor comprises a plurality of reflector assemblies 150 having an outer shape and dimensions similar to the fuel assemblies 34. As indicated in Figures 20, 21 and 22, wherein one such reflector assembly 150 is shown in detail, the principal componentof the reflector assembly is a 37 /3 inches long beryllium block 210 which is solid except for the aforementioned inch diameter central coolant hole 98. The middle 34 /8 inches of the block has outer dimensions and shape substantially identical with the main central section of the fuel assemblies 34 and the remaining 1% inch sections 211 at each end have a reduced circular cross section 2.00 inches in diameter. Upper and lower hollow aluminum end connectors, 212 and 213, respectively, corresponding to the fuel assembly end connectors 68 and 69, respectively, shown in Fig. 8, are attached to the reduced circular end sections 211 of the beryllium block so that the entire reflector assembly 150 may be secured, in the same manner as the fuel assemblies 34, between the upper and lower assembly grids 36 and 38, respectively. Thus the circular upper end connectors 212 are adapted to snugly fit the circular apertures 64 of the upper assembly grid 36, and the rectangular lower end connectors 213- are adapted to snugly fit the rectangular orifices 70 of the lower assembly grid 38. When so secured the beryllium of the reflector assemblies 150 is centrally positioned with respect to the horizontal mid-plane of the active portion The'apertures 66 in the upper assembly grid 36 and the orifices 72 in the lower assembly grid 38 are provided. for eight composite control elements 74 which are slidably disposed within the central channel 20 of the reactor. The control elements 74 are journalled within bearings 76 in the upper and lower guide grids 44 and 48, these bearings being square in cross section. The bottom plug 78 at the lower end of the tank 24 supports eight shock absorbers 80 aligned with the bearings 76in the upper and lower guide grids 44 and 48 for the a purpose of absorbing; a portionof the shock caused by the f lli g control elements 74' shouldthe control elements 74 be rapidly released to shutdownthe reactor incase of emergency. 7

The control elements 74 terminate at their upper end in'an armature head 102 adapted to be accommodated and releasably held by an electromagnet in the form ofa socket 157 (see Figure 3) which-fits over the armature head. The socket issupported by a vertical lifting shaft 158 which extends up through a guide 159 in a spider 160, and thence through the top plug 122 of the tank 24. -The spider 160 is suspendable from top plug 122 via support tubes 170, but it is properly aligned and receives its actual support when in place in the reactor from a ringsupport 171. The lifting shafts 158 and their attached control elements 74 are individually positionable in the vertical .direction by suitable power motors (not shown) locatedon top of plug 122. In case of an emergency requiring the reactor to be shut down as quickly as possible, the currents to all electromagnet sockets 157 are interrupted, whereupon the control elevertically adjacent thereto, another vertically extending portion containing a slow neutron absorber, either of these portions being disposable, by appropriate vertical positioning of the control element, alongside of the fuel containing portion of. the adjacent fuel assembly'34.

The second type of control element 84 has one vertically extending portion containing fissionable material, and vertically adjacent thereto, another vertically extending portion containing a slow neutron absorber, either of these portions being disposable, by appropriate vertical positioning of the control element, alongside of the fuel containing portion of the adjacent fuel assemblies. In the case of both types of control rods 82 and 84, the portion containing the slow neutron absorber is the upper portion.

The firs-t type of control element 82, which may be referred to as a reflecting control element, and which comprises a preferred embodiment of the present invention, is specifically illustrated in Figures 11 and 12. There are four of these control elements 82 and they are arranged to slide in. those four apertures 66 of the upper assembly grid 36 which lie in the second row of apertures from the top (looking atv Figure 7). Each of control elements 82 is provided with a tapered bottom tip 86 which is con-.

structed of materials which will tend to absorb the shock caused by rapid dropping of the control element 82 into shock absorber 80. A plug 88 of shock resistant material sheathed in a jacket 90 of durable material provides. a

tip 86 which will withstand considerable shock. A sleeve 92 is provided with a water outlet aperture 94, and is attached to an element 96 which is formed of a material having neutron moderating properties approximately the same as those of the first reflector 22. A coolant hole 98 extends centrally through the moderating element 96. A

second sleeve 100 is attached to the opposite end of the element 96 and connects the element 96 with the armature head 102 of the control element 82. A water inlet aperture 104 is disposed in the sleeve 100, so that water may enter into the sleeve 100, flow through the coolant hole 98, through the sleeve 92 and out of the aperture 94, thus cooling the control element 82. A slow neutron absorbing liner 106 is also disposed within the sleeve 18! The liner 106 should be constructed of a material having a thermal neutron absorption cross section of at least 100 barns, such as cadmium.

The control element 84 of the type which has a portion containing fissionable material is shown in Figures 13 and 14, and is identical with the control elements 82 containing moderator material, except for the region between the sleeves 92 and 100, and similar numerical on two sides and bowed on the other two sides. Figure 6.) Above and below this central section,

designations have been used on the drawings for identical elements of the two types of control elements 74. The region between the sleeves 92 and 100 of the fuel containing control elements 84 comprises an outer jacket 108 surrounding a plurality of fuel plates 110 identical in design and construction to the inner fuel plates 50 of the fuel assemblies 34.

In the particular reactor construction used throughout as an example, the control elements 74 have an overall length of about 158% inches. The bottom tip 86 is about 16 /2 inches long and its plug 88 is formed of lead and its jacket 90 is formed of stainless steel. The moderating element 96 of control element 82 is formed of beryllium and it begins at a point some 46 inches from the bottom and extends for a distance of 27% inches, The central coolant hole 98 is inch in diameter. The fuel plates 110 of control elements 84 are identical with the previously described inner fuel plates 50 of fuel assemblies 34 and have a spacing between adjacent surfaces of 0.118 inch. There are fourteen of these fuel plates 110. Thus, each control element 84 contains approximately 108 grams of U The absorbing liners 106 in both types of control elements 82 and 84 are square in cross section, about 2.25 inches to a side, and are formed of cadmium clad with aluminum. These liners are 30% inches long. The thickness of the cadmitun layer, itself, is about 0.020 inch. Sleeves 92 and and jacket 108 are formed of aluminum. In external cross section both types of control elements 82 and 84 have a central section, indicated at 161, about 54%; inches long which has an outer shape and dimensions corresponding to the main central sections of fuel assemblies 34 and reflector assemblies 150, that is, straight (See the 1 control elements have square outer cross sections so as to be able to snugly fit and slide within the upper and 1 lower bearings 76.

For fine control of the reactor, four circular holes are left in the first reflector 22 adjacent the central channel for accommodation of four regulating rods 75,

as indicated in Figures 6 and 7, these four rods 75 containing a slow neutron absorber. The details of the reg ulating rods 75 for the exemplified reactor are shown in Figures 16 and 17. The rods are 1 /2 inches in diameter and are hollow substantially throughout their length. Their overall length is about 83% inches. They consist of a lower aluminum section 162, a middle absorber section 163, an upper aluminum section 164, and a ball coupling joint 165 at the top. It will be understood that by means of the joint 165, the rod is releasably attached to a vertical lifting shaft (not shown) which extends through a bearing in the spider 160 and through the top plug 122 and is adjustably positioned in the vertical direction by a motor located on top of plug 122, similarly to the vertical lifting shafts 158 associated with control elements 74. The rods 75 are guided by the accommodating holes in the first reflector 22 and by bearings (not shown) provided in the upper and lower support members 42 and 40, respectively. When detached from its vertical lifting shaft, the regulating rod :75 is supported by its collar 77 resting upon the top of ;,the bearing in the upper support member 42;, in which position its absorber section 163 is centered vertically with respect to the horizontal mid-plane of the reactor active portion 10. The absorber section 163 consists of four 21% inches long inner strips 166 of cadmium encased within aluminum tubing 167. Each of the four cadmium strips 166 is 0.020 inch thick and forms 60 of the arc of a 1.35 inch diameter circle, the strips being equally spaced from one another by 30 of are. N o internal cooling of the regulating rods 75 is required. While provision is made for four of these regulating rods 75, normally only one will be used for control with one more as standby. The remaining two holes in the first reflector 22 can be filled with beryllium rods or canbe used for irradiation of samples.

The unusual flexibility of possible arrangements Within the central channel 20 afforded by the present design may be seen by reference to Figure 15. The entire central channel 20 may be considered as composed of 45 subsections arranged in rows A through E (from top to bottom in Figure 15), each row having subsections a through i (from left to right in Figure 15). Insofar as the reactor structure itself is concerned, each of the subsections excepting subsections b, d, f and h of rows B and D, may be occupied either by a fuel assembly 34, or by a reflector assembly 150, or by neither, in which latter case the subsection is occcupied by water which is also an eflicient moderator. Each of subsections b, d, f and h of row B may be occupied by either the moderator portion or the absorber portion of the associated control element 82 of the present invention, and each of subsections b, d, f, and h of row D may be occupied by either the fuel portion or the absorber portion of the associated control element 84. Of course, there is imposed on the permissible arrangements of fuel assemblies 34 and reflector assemblies 150 the requirement that the number and arrangement of fuel assemblies be suflicient to constitute a critical active portion.

Although not shown in the schematic diagram of Figure 15, it will be understood that the various adjacent fuel assemblies 34 (reflector assemblies 150, and control elements 82 and 84 in the central channel 20 are spaced somewhat from one another and from the channel sides to avoid possible interference and to permit the flow of coolant water therebetween, this spacing amounting to about 0.118 inch in the left-to-light direction of Figure 15, and about 0.039 inch in the up and down direction of Figure 15.

The particular loading pattern of fuel assemblies 34 and reflector assemblies 150 which is employed in the reactor used as a specific example throughout this application is as indicated in Figure 15. As there shown, all subsections of row A and subsections a, c, e, g and i of row B are occupied by reflector assemblies 150, and all subsections of row D, as well as subsections a, c, e, g and i or rows C and E are occupied by fuel assemblies 34. When the reflecting control elements 82 of the present invention and the fuel containing control elements 84 are all the way down, that is, when they are resting in shock absorbers 80, their neutron absorbing liners 106 occupy subsections b, d, f, and g of rows B and D, and these liners extend throughout the entire vertical dimensions of the active portion 10, that is, the liners are centered vertically with respect to the fuel plates 50 of the fuel elements 34. Such a situation would represent the lowest reactivity position of the control elements 74. Under such conditions, the exemplified reactor would be sub-critical and could not support a self-sustaining chain reaction of neutron induced fission. Gradual withdrawal of any one of the control elements 74 would gradually increase the reactivity of the reactor, and the maximum reactivity position of the control elements 74 would be represented by their all being totally withdrawn, that is, by the moderator portion of the control elements 8-2 and the fuel portion of control elements 84 being all centered vertically with respect to the active portion 10. With the control elements in such position, the fuel plates 110 of control elements 84 and the beryllium element 96 of control elements 82 would extend throughout and adjacent, respectively, the entire vertical dimension of the action portion 10. It will thus be apparent that there is provided a large operating range of reactivity between minimum and maximum which may be controlled by appropriate positioning of control elements 82 and 84. At any given time in actual operation, of course, the control elements 82 and 84 will be positioned so that the effective multiplication factor (k sometimes referred to as the reproduction factor (r), for the reactor as a Whole will be 11 sufliciently close to unity that it may be brought exactly to unity by appropriate movement of the regulating rod 75; The actual position of the control elements which will effect this condition will vary, depending upon the' extent of depletion of the uranium 235 in the fuel assemblies 34, the amount of fission products which have accumulated, particularly Xe and iSm the number and character of the irradiations being carried on, the operating temperature, and so forth. It is to compensate for these variables that the large control range of reactivity is provided in the present reactor.

It will be apparent that as the absorber section of the reflecting control element 82 is gradually removed from the reflector region (by upward withdrawal thereof), the moderator section of element 82 immediately and proportionately occupies the space previously occupied by the absorber section. Similarly, as the absorber section of control element 84 is removed from the active portion, the thereby vacated space is immediately occupied by the fissionable section of element 84. It will be appreciated that, as opposed to the conventional type of control element having only a single section of absorber material which leaves a vacant channel upon its removal, control elements 82 and 84 have the advantage that no streaming of neutrons along a vacant control channel occurs upon the removal of the control element, and this advantage, carries with it an additional increase in the range of reactivity control provided by the control element.

In the initial loading of the reactor, top plug 122 to gether with the spider 160 suspended therefrom are removed, as also is the unit consisting of the upper guide grid 44, grid spacer 46, and upper assembly grid 36. All control elements 74 and regulating rods 75 are then placed in their lowermost (minimum reactivity) position, as a precaution to insure against the reactor becoming critical prematurely. The absence of the water moderator from tank 24 in this initial loading step, however, also insures against the reactor becoming critical. The fuel assemblies 34 and reflector assemblies 150 are then inserted with their lower rectangular connectors 69 snugly fitting into the rectangular orifices 70 of the lower assembly grid 38. The upper grid unit is then lowered into place in the upper grid support member 4 2 and locked in position by locking mechanism 16-8, care being taken during the lowering of this unit that the protruding upper ends of the fuel assemblies, reflector assemblies, and control elements are aligned with their various associated accommodating apertures and bearings in the upper grid unit. The top plug 122 and spider 160 are then replaced, and the connections are effected between the tops of control elements 74 and regulating rods 75 and'their respective associated vertical lifting shafts 158. After the water and air coolant systems are placed in operation, the reactor may be brought to a critical condition and its power raised to the design level by gradual withdrawal of one or more of the control elements 74. Short term variations of the power level near its operating level may thereafter be controlled by means of regulating rods 75.

After initial operation, fuel assemblies, reflector assemblies, control elements, and regulating rods may be removed from the top of tank 24, with water in the tank, by a reversal of the above-described process provided these parts are not too radioactive. However, after the reactor has been operating at design power level for some time, the fuel assemblies and the control elements, particularly those control elements of type 84 having a fuel portion, will be too radioactive to be unloaded from the top of the reactor. For unloading these highly radioactive fuel assemblies and control elements, a discharge chute 172 (Figure 4) is provided, the chute being aligned with a removable beryllium plug 174 passing vertically through, and forming a part of, the first reflector 22. Chute 172 passes through bottom plug 78, and leads to a canal 173 (see Figure 1) via a valve and seal (not shown) which acts as a lock to prevent excessive loss of water during unloading. sembly, for example, the top plug 122, the spider 160 suspended therefrom, and the unit consisting. of upper.

guide grid 44, grid spacer 46 and upper assembly'grid 36-, are removed upwardly from the tank 24'. The beryllium plug 174 is then temporarily withdrawn upwardly,

It will be understood that elongated handtools (not shown) in the nature of grappling devices may be employed to reach down into the tank 24 from above to grasp and manipulate the fuel assemblies, control elements, and removable beryllium plug. Itthen passes via chute 172 into the canal 173 along which it can be conveniently removed from the immediate vicinity of the reactor while always being maintained under about 15 feet of water. 7

The reactor used throughout as a specific example is designed to operate at a power level of 30,000 kw. of heat and an average slow neutron flux level in the active portion of about 2 10 neutrons/cm. sec. The 30,000 kw. of heat is distributed approximately as follows: 28,250 kw. in the active portion, 1,200 kw. in the preliminary and first reflector, 500 kw. in the graphite second reflector, and 50 kw. in the thermal shield 28. This heat is removed by a primary water coolant system and a secondary air coolant system, as previously indicated. The water coolant system uses recirculating demineralized water which flows through the tank 24 at a rate of 20,000 gallons/min, entering at F. and leaving at 111 F. The water velocity through the spaces between the fuel plates of the fuel assemblies 34 is about 30 ft./ sec. The fuel plates represent a total active surface area of about 400 square feet. A pressure drop of about 40 p.s.i. is experienced by the water in flowing through the reactor. Under such conditions, the average heat flux in the active portion is about 300,000 Btu/square ft./hr. and the maximum heat flux at any point in the active portion is about 500,000 Btu/square ft./hr. With such a water coolant system, the maximum temperature of any metal surface within the tank 24 will not exceed 212 F. that is, the boiling temperature of the water coolant.

The secondary air coolant system consists of a flow of about 2000 lbs/min. at a pressure drop of about 55 inches of water, about 16 10 lbs./ min. flowing in the path including the side thermal shield 28 and second reflector 26, about 250 lbs/min. flowing in the path including the top thermal shield 28, and about lbs/min. being allowed for leakage through and along the various irradiation passages 32. The air enters at about 80 F. and leaves at about F. Such an air coolant system will serve to maintain the temperature of all parts located outside tank 24 at a safe value with an adequate margin of safety.

It should be stated that the heat evolved in the reactor cannot be reduced to zero instantaneously simply by inserting all control elements and thus shutting down the reactor. This is due to the continued heat generation resulting from absorption of the gamma and beta radiation from the radioactive fission products. Thus, immediately after shut down, the active portion will still? generate about 2000 kw. and the beryllium reflector about 400 kw. and these values will thereafter fall off proportionally to t-- wherein t is time in seconds. Thus, the water coolant must continue to be circulated at a minimum of about one-third normal rate immediately after shut down, and at a minimum of about one-fortieth normal rate 10 hours after shutdown. Similarly, the air coolant system should be kept in operation for some time aft-er shut down.

Some of the nuclear physics aspects of the reactor will now be dealt with. In the interests of definiteness, it will be considered for the moment that the four control elements 84 in row D of Figure 15 are fully with- In order to unload a fuel as asaeec drawn so that their fuel portions are centered with respect to the active portion, and that the active portion is in a cold clean state, that is, it is operating at zero power with unused fuel assemblies 34 and control elements 84. (Undepleted fuel and no accumulated fission products.) The active portion composition then may be defined generally as one consisting in its entirety of water, aluminum, U and U having a U concentration of 38 gms./liter; having a volumetric ratio of aluminum to Water of 0.71; and having an atomic ratio of U to U of 143. Such a reactive composition has a multiplication constant (k) of about 1.61; a Fermi age (r) of about 62 cm. a diffusion area (L of about 3.4 cm. and a thermal utilization factor (f) of about 0.75. When surrounded by a berryllium reflector, as in the present case, such a reactor composition would become critical at dimensions substantially less than the actual dimensions of the active portion of the present reactor. Thus, the herein exemplified reactor has a large amount of excess reactivity built into it, and this excess reactivity is initially held or cancelled out by means of control elements 74.

This excess reactivity is required in order to compensate for four poisoning efiects involved in the actual continued operation of the reactor. The first of these is depletion of the U during operation, which, at the design power level, takes place at a rate of about 1% per day. Since it is desired to operate on a 15 day fuel assembly recharging cycle, a total of 15% depletion will occur in the fuel assemblies before they are replaced.

A second poisoning effect arises from the build-up of various fission products which parasitically absorb neutrons. During operation of the reactor, the concentration of the various fission products tends to attain an equilibrium value at which their rate of formation, directly from fission or from radioactive decay of parent fission products, is exactly equal to their rate of disap pearance as a result of radioactive decay and nucleai transformation. Furthermore, two of the most troublesome fission products Xe and Sm are daughter products of the short half-life fission products I and H respectively. The concentration of these fission products Xe and Sm therefore, continues to build up after the reactor is shut down, attaining a maximum about hours after shut down and thereafter decreasing. Since it would require an undue amount of excess reactivity to be built into the reactor in order to compensate for the maximum possible loss in reactivity from Xe and S111 at any time after shut down (about 43% .10 hours after shut down), a compromise has been effected in the present reactor whereby sufficient excess reactivity (about 9.6%) is provided to override the Xe and S111 effect either within one-half hour after shutdown or more than 48 hours after shutdown. This, of

course, imposes a premium upon the ability of the opk i inally, a forth reactivity loss of about 5% will be involved in the introduction of samples into the various irradiation channels of the reactor.

The various reactivity losses mentioned above may be summarized as follows:

Source of reactivity loss:

In order to be able to override the above-indicated reactivity losses, the present reactor has built into it an excess reactivity of about 19% The eight control elements 74 are capable of controlling a total of about 40% and each regulating rod 75 is capable of controlling about 0.5%

With all control elements and regulating rods in their in, or minimum reactivity, positions, the effective multiplication factor k of the reactor in its cold, clean state is about 0.8.

At power levels below about 10- of design power, that is, at slow neutron flux levels below about 2000 neutrons/om. /sec., no accurate and reliable means for measuring and indicating the slow neutron flux is provided. This, therefore, is a blind region in which the operator would not be permitted to withdraw any of the control elements 74 at all, lest the reactor became supercritical on a very short period without his knowledge. The slow neutron flux of the reactor must therefore be raised above this blind region by means of some extraneous source of neutrons. In prior reactors, provision has been made for the removable insertion into the reactor of a conventional radium-beryllium or polonium-berylli-um source for this purpose.

In the present case, advantage is taken of the photoneutron sensitive properties of the beryllium already present in the reactor to provide a permanently incorporated and automatically reactivated neutron source. For this purpose, a quantity of Sb is permanently incorporated in the reactor in the vicinity of the reactor beryllium. During operation of the reactor, some of the Sb atoms undergo a nuclear transformation by slow neutron absorption and become radioactive, the nuclear reaction being as follows:

The resulting Sb is radioactive, emit-ting a 1.72 mev. gamma ray with a 60 day half-life. Since berylliurn is photo-neutron sensitive to gamma photons of this energy, neutrons are produced by interaction of the gamma rays and the beryllium, according to the following nuclear reaction:

The threshold energy of the incident gamma photon for the immediately above reaction is about 1.64 mev., which is below the 1.72 mev. energy of the gamma ray given oif by the Sb atoms. Thus, by the simple etc. at various points in the reactor.

permanent incorporation of a quantity of Sli in the vicinity of the beryllium, a cheap, convenient, permanent, and automatically reactivating neutron source is provided of sufficient strength to maintain the slow neutron flux level at a sufliciently high level to be reliably measured for the duration of periods of shut down. In order to provide for the very first start-up of the reactor, the antimony Which is to be permanently incorporated in the reactor may be made initially radioactive by subjecting it to neutron irradiation in another reactor. Alternatively, a conventional radium-beryllium or poloniurn-beryllium' neutron source may be used for the very first start-up.

In the specifically exemplified reactor, the antimony is in the form of a solid cylindrical vertical rod one-half inch in diameter and six inches long. It is formed entirely of normal or natural occurring antimony, the Sb isotopic component thereof being innocuous. It is positioned in the first reflector 22 at a point in the horizontal plane which is schematically indicated by reference numeral 15 in Figure 7, there being a vertical hole in the first reflector to accommodate it. In the vertical direction it is centered with respect to the horizontal mid-plane of the active portion 10. This amount of antimony will last indefinitely, and after a short period of full power operation of the reactor, will have become sufl'iciently radioactive to maintain the slow neutron flux level of the reactor above 2000 neutrons/cmP/sec. for many days after the reactor is shut down. It should be pointed out that the actual position of the antimony in the reactor is not at all critical, but for maximum effectiveness, of course, it should be located reasonably near both the reactor beryllium and the reactor active portion.

It will be understood that in order to provide the operator with information required in the control of the reactor, various types of conventional instruments are provided to measure such operating data as neutron flux, reactor period, temperature, pressure, coolant activity, Automatic safety circuits, responsive to various of these measurements, are also preferably provided, these circuits being adapted to cut off the electromagnet holding currents and thus release the control elements 74 and shut down the reactor in the event that any of these measurements should indicate an unsafe operation condition. While the present reactor can, with some degree of ditficulty and inefliciency, be operated entirely under the control of the operator, assisted, if desired, by various types of automatic control devices and circuits of the prior art, in the interests of efliciency, it is preferableto employ the combination manual and automatic control system especially developed for the exemplified reactor, which system is described and claimed in the co-pending U.S patent application Serial No. 357,216, of common assignee, entitled Overall Control System for High Flux Pile, filed on May 25, 1953, in the names of Henry W. Newson, Eugene P. Wigner, Elbert P. Epler, and Thomas E. Cole.

As stated above, the present reactor has the unusual property of having a higher maximum slow neutron flux in the reflector then in the active portion, or core, of the reactor. In order to have an accumulation of slow neutrons in the reflector of a reactor which exceeds that in the core of the reactor, certain limitations must be placed upon the construction of the reactor. In the' first place, it is clear that the absorption of slow neutrons in the reflector of the reactor must be relatively small, and. also that the adsorption of slow neutrons in the core of the reactor must be relatively high. Hence, not all neutron reflecting, or moderating, materials may be used in the reflector of the reactor, nor can all types of reactive compositions be utilized in the core. It has been found that the maximum slow neutron: flux in the reflector of the h reactor will exceed the'maximum slow neutron flux in the core when the ratio of thethermal neutron scattering cross section to thermal neutron absorption cross section,

15 averaged overall of the materials in the reflector, is approximately twice the ratio of the thermal neutron scattering cross section to thermal neutron absorption cross section, averagedover all of the materials in the core of the reactor, and that this ratio is preferably in the neighborhood of 10 for such a reactor. For a reactor having the particular reactive core composition employed in the exemplified design, a reflector formed essentially of either beryllium or heavy water will satisfy this requirement.

It is also necessary that the reflector region which conforms to the above requirement be of sufiicient thickness and 'be located immediately adjacent the core. It has been found that a reflector of about 12 inches thickness is adequate when formed essentially of beryllium. It will be noted that in the present design these requirements are met in all lateral directions but not in the vertical direction, since more than 12 inches of beryllium immediately surrounds the core on all sides, whereas ordinary water is employed as the reflector immediately above and below the reactor. It should be pointed out that it is necessary to meet the above indicated requirements only with respect to the side or sides of the core in which it is desired to achieve a higher slow neutron flux in the reflector than in the core. V

In order to have an accumulation of slow neutronsin the reflector, it is also necessary that the core of the reactor have one overall dimension which is not more than one-half of the other overall dimensions of the core, and preferably not more than one-fifth of the other dimensions. in the particular reactor described to illustrate the present invention, the overall dimensions of the core of the reactor are approximately 9 /2 inches wide by 28 inches long by 23% inches high, thus clearly meet ing this requirement. It is also to be noted that the core of this reactor is in the form of a thin slab, which is believed to be the optimum geometry for a reactor achieving these desired results. I

It is possible to determine whether a particular neutronic reactor will exhibit the property of having a higher maximum slow neutron flux in the reflector than in the core by means of the equation:

mf M where N is a number which will be referred to as the neutron distribution factor of the reactor, k is the multiplication constant of the reactive composition employed asthe core of the reactor, M is the migration area for neutrons in the reflector of the reactor, and M is the migration area for neutrons in the core of the reactor. It has been found that forvalues of N in excess of 9, the maximum slow neutron-flux in the reflector of the reactor will exceed the maximum slow neutron flux in the core of the reactor.

It is evident from the foregoing discussion, that the construction of a neutronic reactor hating a maximum slow neutron flux in its reflector greater than the maximum slow-neutron flux in its core involves the use of a reactive core composition having a high multiplication constant (k). It has been found, that neutronic reactors, wherein the multiplication constant (k) of the core composition is less than 1.3, cannot be made to attain criticality with. the relative core dimensions'required for it to exhibit. a higher maximum slow neutron flux in the reflector of the reactor than in the core thereof.

The particular reactor design employed as a specific example throughout the'present application meets all of the above requirements, With the result that the maximum slow neutron flux in the beryllium reflector exceeds the maximum slow neutronflux in the core. This is indicated in Figure 18 wherein the profiles of various slow neutron flux levels in the horizontal mid-plane of the exemplified reactor, operating at its design power level, are shown. In this figurethe numbers within the boxes designate'the slow neutron flux of the associated profile in units of 10 neutrons/cmP/sec. As indicated in Figure 18, the maximum slow neutron flux occurs in the beryllium-reflector several inches from the center of each long side of the core and has a value of about x neutrons/cm. /sec. The maximum slow neutron flux in the core 10 occurs at its center and has a value of about 2.5 10 neutrons/cmF/sec, the average slow neutron flux in the core being about 2X10 neu trons/cm. /sec. In the vertical direction,- where ordinary Water is the reflector immediately adjacent the core, the conditions required to achieve this effect are not met, and the slow neutron flux distribution taken along the vertical center line of the core exhibits its absolute maximum at the center of the core. The fast neutron flux has its maximum value of about 4.0 10 v neutrons/ cmF/sec. at the center of the core and continuously falls offin all directions outwardly from the center. It will be evident from the symmetry of the flux pro-' files about the core 10, as illustrated in Figure 18, that the neutrons recognize no distinction between what has herein been referred to as the preliminary reflector 12 on one side of the core 10, and that portion of what has herein been referred to as the first reflector 22 which lies on the opposite side of the core. They both serve as neutron reflectors for the core and both have substantially the same nuclear characteristics. In the description of the actual construction of this reactor, however, it has been found convenient to distinguish between them as to nomenclature since their components differ structurally. It should not be implied from this difference in nomenclature that the two reflectors serve different nuclear purposes or have different nuclear effects. Another example of a reactor meeting the conditions required to exhibit a higher maximum slow neutron flux in the reflector than in the core comprises a rectangular core 51 cm. 66 cm., consisting in its entirety of a substantially homogeneous mixture of U U aluminum, and water; containing 1.35 kg. of U having a volumetric ratio of aluminum to water of 0.65; and having an atomic ratio of U to U of 14.3; the core'being immediately surrounded with a 30 cm. beryllium reflector. The fuel assemblies 34 constitute a most difficult and critical element in the design and operation of the present reactor. It will be apparent that the fuel plates 50 are subject to intense irradiation by fast neutrons, fission fragments, and other radiation, all of which has-a deteri orative effect on the internal structure 'and physical characteristics of the plates. These plates are also subject to the thermal stresses induced by the temperature varia tions over their operating temperature range and to the strains and pressures resulting from the high rate of flow of coolant water. At the same time, it is desired that their lifetime be such that as much as of their total U content placed. All of these factors necessarily tend to produce dimensional changes, distortion and warping of the. plates. On the other hand, it is most important that the spacing between plates be maintained essentially uniform over the entire large surface area of the plates in order that the efliciency of heat removal not be impaired and local and tolerable temperature rises result. Complicating the problem still more is the fact that the usual mechanical aids in the maintenance of uniformity of spacing, such as the comb-shaped supports 62 and the grooves in side plates 52, can only be applied at the plate edges, that is, where there exists no inner layer of fissionable material requiring cooling.

This seemingly almost hopeless problem was solved tions and thus approach or actually contact each other. Another advantage derived from the initial curvature of the plates is a geometrical one which derives from the fact that any given amount of thermal expansion of a curved plate having its ends fixed results in less actual lateral displacement at the center of the plate than would be the case if the plates were flat. It has been found that the use of curved parallel fuel plates, as described, avoids any serious adverse effects resulting from the distortion and warping inherent under the stringent operating conditions involved in the present design.

While the actual position of the reflecting control elements 82 of the present invention in the reflector is not crucial, they are most effective and will provide the greatest range of reactivity control if they are located relatively near the active portion and in the vicinity of the maximum slow neutron flux of the reflector region. It will be apparent by reference to Figures 15 and 18 that, in the illustrated reactor, the control elements 82 are located, as is preferred, not only immediately adjacent the active portion 10, but also in the portion of the reflector region characterized by its highest slow neutron flux. It will be apparent that the present invention is uniquely ad vantageous in association with a reactor of the type illustrated having its maximum slow neutron flux in its reflector rather than in its core.

It will be appreciated that the reflector control element 82 of the present invention is not necessarily limited in its application to reactors generally to movement through the reflector. One conventional type, of reactor has an active portion consisting of a fairly coarse matrix of fuel and solid moderator. As applied to such a reactor, for example, the control element 82 could be positioned directly in the active portion for movement in a control channel extending through the solid moderator ofthe active portion. v

In the construction of the present reactor it is, of course, desirable that the various non-fissionable materials employed in and near the core have as high a density as possible and contain as few impurities as possible, particularly impurities such as boron, having high thermal .neutron absorption cross sections. However, since the present reactor employs uranium highly enriched in the fissionable 235 isotope, and has considerable excess reactivity built into it, the density and purity of-rnaterials is not crucial, as it is in natural uranium reactors, the only adverse effect of low density materials including impurities being to reduce somewhat the excess reactivity of the reactor and thus to decrease its operating undergo fission before they are reby giving all of the plates an identical initial curvature in onedirection, as shown in Figure 9. This assures that any additional buckling of the plates during operation will act equally, and in the same direction, for all plates, that is, in a direction such as to augment the initial curvature. With the present design therefore, it is impossible for adjacent plates to buckle in opposite direcflexibility. The better grades of commercially available non-fissionable materials are entirely satisfactory for use in the construction of the exemplified reactor.

While various of the'figuresof the drawings are drawn to diiferentscales, individual figures are drawn in their entirety to auniform scale. Accordingly, any dimensions of the specifically exemplified reactor of Figures 1 to 18 not specified in the written description can be obtained from the drawings by comparison with a specified dimension of a part which is shown on the same figure.

In Figure 19, there is shown schematically a generalized form of reactor incorporating in its reflector region several control elements which form variations and 'extensions of the basic principles of the present invention. The reactor is shown as comprising a'central active portion 10' surrounded in all lateral directions by a reflector region 22'. 'The active portion 10' will be understood to' conta.in fissionable material, such as uranium-235, and moderator material, such as beryllium, the atomic ratio of moderator to fissionable atoms being such (usually in the'thousands) as to characterize the active portion as a thermal or slow active portion, that is, an active portion in which thernajority of the fissions are induced by neutrons at slow or thermal energies. The reflector region 22f will .be understood to be formed essentially of amoderator material such as beryllium. 

